“I look forward to this,” Williams said. “It’s almost like a launching pad for what I want to do for the rest of the hard-court season.”

In a way, yes, Monday’s start of the year’s last Grand Slam actually marks something of a new beginning — the kickoff of a six-month stretch on the hard courts that winds down at the 2013 Australian Open.

Call it mental gymnastics, a creative way of looking at things or whatever else might apply. What can’t be denied is that in an Olympic year, the U.S. Open — considered the toughest test in tennis even under normal circumstances — is essentially the season’s fifth major. That makes for quite a grueling season for the players.

Indeed, many have had to double down on their fitness and find new, creative ways of organizing their schedules to get ready for what they hope will be a two-week grind in the fishbowl that is Flushing Meadows.

Defending champion Novak Djokovic barely took any time off following his fourth-place finish at the Olympics. He traveled to Toronto for a hard-court tuneup, played six matches and won the tournament.

Then, he flew to Cincinnati and played six more matches but lost to Roger Federer in the final. No shame there, though that loss to Federer did include an uncharacteristic 6-0 whitewashing in the first set.

“Mentally, I wasn’t there, wasn’t fresh,” Djokovic said. “It had been a very busy time starting at the Olympic Games, and maybe that caught up with me at the end.”

No big deal in Cincinnati. But a half-hour mental lapse in New York could mean the end of Djokovic’s quest to win what has, essentially, shaped up as the tiebreaker major for 2012.

Second-seeded Djokovic won the Australian Open. Rafael Nadal won the French Open. Top-seeded Federer won Wimbledon. Just for good measure, third-seeded Andy Murray won the Olympics, meaning the U.S. Open could essentially determine the player of the year in men’s tennis.

Some combination of Nadal — absent this year because of a knee injury — and the other three have occupied every spot in the finals of the past eight Grand Slam tournaments.

“The way it pans out, it’s conceivable that Murray could make an argument were he to win this … that you could say he’s the best player in the world this year,” McEnroe said. “To me, that’s an unbelievable upside.”

Federer, back at the top of the rankings after 25½ months during which Nadal and Djokovic took the spot from him, has a night match Monday against American Donald Young, who is 3-21 this season.

Federer says there’s a difference between how he feels now and last year, when he had two match points in the semifinals against Djokovic, but lost both and closed out his first full season since 2002 without a Grand Slam title.

“I think I felt good last year, but probably felt that maybe, at times, the matches were not always on my racket,” he said. “Whereas maybe this time around, I feel like if I’m playing well, I can dictate who’s going to win or lose.”

Though the women’s game has been more in flux than the men’s of late — seven different winners over the past seven Grand Slams — the math turned out essentially the same in 2012: Three of the top four women — No. 1 Victoria Azarenka (Australia), No. 3 Maria Sharapova (France) and No. 4 Williams (Wimbledon) — have major titles this year and all need this one to break the tie.

Where things differ is in the way Williams has been playing of late. She lost a total of 17 games over six matches in the Olympics, punctuating it with a 6-0, 6-1 victory over Sharapova in the final — the kind of drubbing that would have to come to mind if the two should meet in the U.S. Open final on Sept. 8.

Sharapova had two hard-court tuneup tournaments on her schedule, but pulled out of both with a stomach virus.

“I think it was a sign my body just needed to slow down,” she said. “It was a lot of travel, a lot of playing. I had a hectic summer. So, I decided to shut it down until here, because we still have a lot to play towards the end of the year.”

Kim Clijsters, on the other hand, doesn’t have to save any energy for down the road. Win or lose, she says the U.S. Open will be it for her.

“Here,” she said, while pointing to her heart. “You feel when it’s right.”

Clijsters, who missed last year with a stomach-muscle injury, has won her past 21 matches at Flushing Meadows. In 2005, she won the tournament. She didn’t return again until 2009, after she had gotten married and had a daughter, Jada. With virtually no tournament play under her belt in 2009, Clijsters won seven matches to become the first unseeded woman to capture the U.S. Open. It was her first of back-to-back titles.

Because of how busy 2012 has been, she’ll do the same as Sharapova — come into the U.S. Open not having played a competitive match since the Olympics.

As for defending champion Samantha Stosur? She’s OK with not being among the favorites.

“At this point during the tournament, it doesn’t really matter,” she said. “Everyone is starting from scratch, and obviously things become clearer as the weeks go on.”

Who’s your pick to win the U.S. Open? Make your predictions in the comments below!

(TM and Copyright 2012 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2011 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)